In wide use and possessing various designs in power transmission machinery is the universal joint (sometimes termed knuckle joint) which has been long existant in the art of machine building. In its earliest and simplest forms such is familiarly known as the Hooke's or Cardan type, consisting essentially of two forks connected through an intermediate block straddled by the forks and journalled thereto at right angles to each other. The Hooke's-Carden type of joint has the virtue of simplicity combined with capability of accommodating high torques at very large angles of deflection between input and output shafts. However, the dynamic performance is not ideal, since there is a variation between the angular motions of the connected shafts, which has unacceptably severe inertial load consequences at: high speeds, large deflection angles, or with substantial masses connected by the cooperating shafts. Elementary analysis shows that the angular variation characteristic of the Hooke's or Cardan joint is described by the relationship EQU tan .alpha. = tan .beta. sec .DELTA.,
where .alpha. and .beta. are the angles of rotation of the input and output shafts and .DELTA. is the deflection angle between the shafts. A typical value of the variation is seen at .DELTA. = 36.degree. which is at the upper limit reached in most high speed applications such as automatic front wheel drives, axles and transmission shafts. At this deflection the output shaft will alternately be angularly advanced or retarded, each twice per shaft revolution, to peak values of .+-. 6.degree.. The corresponding variation in velocity is .+-. 24% referred to the input speed.
It is clear that such variations in relative position would be awkward in precision control applications where it is important to maintain accuracy of motion from one point in a machine drive to another point via a universal joint used to change shaft direction. The substantially large cyclical variations in velocity create intolerable vibrations and acceleration loads where massive loads are being driven.
Despite the undesirable characteristic of nonuniformity in the Hooke's or Cardan joint, its simplicity has led to extensive application where speeds and inertial loads are low and demands for precision of position are minimal. But increasing areas of application are developing where the consequences of motion irregularity are unacceptable for reasons of noise, excessive vibration, imprecision of control, or consequent wear. Numerous examples can be cited such as the automotive front wheel drives, helicopter rotor drives, marine inboard-outboard propellor shafts, hydraulic pump swash plate drives, etc.
Beginning with the era of abruptly higher machine and transportation speeds circa the end of World War I, intensive efforts were made to find substitutes (for the Hooke's-Cardan type of joint) which would have uniform velocity performance. A small number of successful joints have been found, notably the Rzeppa and Weiss rolling ball joints, and a variety of sliding block types of which the Tracta appears to be the most frequently used. In addition a number of "kinematic" models have appeared which are theoretical laboratory solutions of the problem of providing constant velocity angular transmission. However, regarding the latter, their practical value is questionable for reasons of complexity, non-compactness, or low-load carrying ability.
A fairly frequent solution is the use of two Hooke's-Cardan joints in series, so phased that the irregularities of one are cancelled out by oppositely directed variations of the other. This is less than ideal because of: the added space required by the second joint, the irregular motion of the intermediate member between the two, inability in many applications to ensure that each joint operates at the same angle as the other; as well as other obvious disadvantages such as cost, noise vibration and wear to mention a few.
The rolling ball joints invented almost 50 years ago have enjoyed the greatest success and are used currently as first choice where high performance is required. However, they have limitations in respect to high manufacturing cost, limited durability, and are subject to derating at high speeds and large deflection angles. They are not susceptible of adjustment to take up wear and therefore cannot be assembled to a true zero backlash condition (in view of manufacturing tolerances) without preloading which detracts from load carrying ability and economy of production.
It may therefore be fairly said that much room for improvement remains in the evolution of the universal joint regarding economy, simplicity, durability and other factors.